A digital communication system 100 can be modelled as illustrated in FIG. 1. Data information is modulated to message symbols mi taken from a symbol alphabet M={m0, m1, . . . , mM−1}. One common multilevel modulation method is Quadrature Amplitude Modulation (QAM). A digital transmitter based on QAM maps data onto a 2-dimensional (N=2) coordinate system of orthogonal base vectors. FIG. 2 shows an example of 16-QAM (M has 16 elements) with one out of many possible numbering schemes. The message symbols are converted into vectors of real numbers,
      s    i    =      {                                        s                          i              ⁢                                                          ⁢              0                                                                        s                          i              ⁢                                                          ⁢              1                                                            ⋯                                                  s                          iN              -              1                                            }  where the dimension is N≦M, by a vector transmitter 101. A modulator 102 constructs a signal Si(t) which is sent over a channel 103. The channel affects the signal si(t) and creates a signal x(t). A detector 104 receives the signal x(t), demodulates the signal and constructs an observation vector x. The observation vector is decoded by a vector receiver 105 producing an estimate {circumflex over (m)} of the symbol being sent.
Digital communication requires that there exist a method for synchronizing the receiver to the transmitter in order to decode the data correctly. The channel over which the communication takes place normally also introduces a linear transformation of the signal, which leads to the requirement of an equalization method. Synchronization and equalization methods vary between different communication systems, however, the methods are often developed under the assumption that the transmitted data is randomized in some way, usually by scrambling the data prior to modulation with a pseudo-noise sequence that is also known at the receiver. The scrambler ensures that even if the incoming data consists of long sequences of constant patterns, the data over the channel is randomized. One particular situation where long sequences of constant patterns normally occurs in the data stream is when there is no user data, i.e., the transmitter has to fill the data channel with dummy data. The scrambling ensures that the power level on the channel is approximately constant regardless of the data content user data or dummy data. The average energy per modulation interval is
                              E          av                =                                            1                                              M                                                      ⁢                                          ∑                                  i                  =                  0                                                                                          M                                                        -                  1                                            ⁢                              E                i                                              =                                    (        1        )            where Ei is the energy level for a particular constellation point and ∥M∥ is the size of the symbol alphabet which is equal to the number of constellation points.
In contrast, in several applications, such as Very high data rate Digital Subscriber Line (VDSL) system, there would be desirable to be able to transmit with a lower power when there is no user data to be sent. Depending on the type of application, this can result in reduced crosstalk, reduced radio frequency interference, and/or reduced power consumption in line driver circuitry.
Existing solutions require that the transmitter and receiver agrees upon entering a special power reduction mode when the transmitter predicts that user data will be absent for some period of time. A multi-carrier specific solution is disclosed in EP 883269 A1.
However, it is often difficult to predict the behaviour of user data. Entering and leaving a special power reduction mode requires communication between the transmitter and receiver and is therefore time consuming. The period of time that user data will be absent must be longer than the time it takes to reach an agreement between transmitter and receiver. During the power reduction mode, the transmitter and receiver are still connected, but a transition to the power reduction mode implies that the transmitter performs the following in order to reach the agreement with the receiver:                The transmitter requires information if no data will be transmitted in the immediate future. This information is often based on measurements and assumptions.        Determine the exact point of time for the transition into the power reduction mode and communicate the point of time to the receiver.        Receive an acknowledgement from the receiver.        
A transition from the power reduction mode implies that the transmitter performs the following:                Determine an exact point of time for the transition from the power reduction mode and communicate the point of time to the receiver.        Receive an acknowledgement from the receiver.        
Therefore, these solutions do not work when user data and dummy data are heavily interleaved. Entering and leaving the power reduction mode also leads to communication overhead in terms of an increased signalling.
It is also desirable that the values of the dummy data is randomized in order to avoid e.g. major peaks in the output signal.